The invention relates to N protein fusion proteins of interest, possibly in the form of N protein protein of interest/P protein soluble complexes, the N and P proteins being proteins of a virus in the Paramyxoviridae family. When the protein of interest is an antigen, the invention also relates to vaccine compositions and reagents of the diagnostic comprising these N protein-antigen fusion proteins or these N protein-antigen/P protein complexes. The N protein-protein of interest fusion may also be used as a 'vector' to transport therapeutic molecules of interest in the cells, such as antiviral or anticancer molecules.